A variety of problems associated with the formation and discharge of electrostatic charge during the manufacture and use of photographic films are well recognized in the photographic industry. These electrostatic charges are generated by the highly insulating polymeric film bases, such as polyester and cellulose acetate, during winding and unwinding operations associated with the photographic film manufacturing process and during the automated transport of photographic films in film cassette loaders, cameras, and film processing equipment during use of the photographic film product.
It is well known that electrostatic charges can be effectively controlled or eliminated by incorporating one or more electrically-conductive antistatic layers in the photographic film. A wide variety of conductive materials can be incorporated into antistatic layers to provide a wide range of conductivity and antistatic performance. Typically, the antistatic layers for photographic applications employ materials which exhibit ionic conductivity where the charge is transferred by the bulk diffusion of charged species through an electrolyte. Antistatic layers comprising inorganic salts, ionic conductive polymers, and colloidal metal oxide sols stabilized by salts have been described. U.S. Pat. No. 4,542,095 discloses antistatic compositions for use in photographic elements wherein aqueous latex compositions are used as binder materials in conjunction with polymerized alkylene oxide monomers and alkali metal salts as the antistatic agents. U.S. Pat. No. 4,916,011 describes antistatic layers comprising ionically conductive styrene sulfonate interpolymers, a latex binder, and a crosslinking agent. U.S. Pat. No. 5,045,394 describes antistatic backing layers containing Al-modified colloidal silica, latex binder polymer, and organic or inorganic salts which provide good writing or printing surfaces. The conductivities of these ionic conductive antistatic layers are very dependent on humidity and film processing. At low humidities and after conventional film processing the antistatic performance is substantially reduced or ineffective.
Antistatic layers employing electronic conductors have also been described. The conductivity of these materials depends on primarily electronic mobilities rather than ionic mobilities and the conductivity is independent of humidity. Antistatic layers which contain conjugated polymers, semiconductive metal halide salts, conductive carbon or semiconductive metal oxide particles have been described. It is characteristic of these electronically conductive materials to be highly colored or have high refractive index. Thus, providing highly transparent, coloress antistatic layers containing these materials poses a considerable challenge.
U.S. Pat. No. 3,245,833 describes conductive coatings containing semiconductive silver or copper iodide dispersed as 0.1 .mu.m or less particles in an insulating film-forming binder exhibiting surface resistivities of 10.sup.2 to 10.sup.11 .OMEGA. per square. However, these coatings must be overcoated with a water-impermeable barrier layer to prevent the loss of conductivity after film processing since these semiconductive salts are solubilized by conventional film processing solutions.
Conductive layers comprising inherently conductive polymers such as polyacetylene, polyaniline, polythiophene, and polypyrrole are described in U.S. Pat. No. 4,237,194, JP A2282245, and JP A2282248, but, these layers are highly colored.
Conductive fine particles of crystalline metal oxides dispersed with a polymeric binder have been used to prepare humidity insensitive, conductive layers for various imaging applications. Many different metal oxides are alleged to be useful as antistatic agents in photographic elements or as conductive agents in electrographic elements in such patents as U.S. Pat. Nos. 4,275,103, 4,394,441, 4,416,963, 4,418,141, 4,431,764, 4,495,276, 4,571,361, 4,999,276, 5,368,995 and 5,457,013. Preferred metal oxides are antimony doped tin oxide, aluminum doped zinc oxide, niobium doped titanium oxide, and metal antimonates. These patents do not teach any specific requirements for the binder polymer and, as a result, there is a need for a high volume % of the conductive fine particles in the conductive coatings in order to achieve effective antistatic performance. This results in reduced transparency due to scattering losses and in brittle films subject to cracking and poor adherence to the support material.
JP A4055492 describes antistatic layers comprising conductive non-oxide particles including TiN, NbB.sub.2, TiC, and MoB dispersed in a binder such as a water soluble polymer or solvent soluble resin.
U.S. Pat. No. 5,066,422 describes vinyl surface covering materials comprising a fused sheet of a dry blend, wherein the dry blend contains a polyvinyl chloride porous resin, a plasticizer, and conductive particles. Reportedly, the conductive particles reside in the pores and surface of the polyvinyl chloride resin which thereby provides surface resistivities of the fused sheet of 10.sup.9 .OMEGA. per square at low weight % of the conductive particles.
Fibrous conductive powders comprising antimony doped tin oxide coated onto nonconductive potassium titanate whiskers have been used to prepare conductive layers for photographic and electrographic applications. Such materials have been disclosed in U.S. Pat. No. 4,845,369, U.S. Pat. No. 5,116,666, JP A63098656, and JP A63060452. Layers containing these conductive whiskers dispersed in a binder reportedly provide improved conductivity at lower volume % than the aforementioned conductive fine particles as a result of their higher aspect (length to diameter) ratio. However, the benefits obtained as a result of the reduced volume % requirements are offset by the fact that these materials are large in size (10 to 20 .mu.m long and 0.2-0.5 .mu.m diameter). The large size results in increased light scattering and hazy coatings.
Transparent, binderless, electrically semiconductive metal oxide thin films formed by oxidation of thin metal films which have been vapor deposited onto film base are described in U.S. Pat. No. 4,078,935. The resistivity of such conductive thin films has been reported to be 10.sup.5 .OMEGA. per square. However, these metal oxide thin films are unsuitable for photographic film applications since the overall process used to prepare them is complex and expensive and adhesion of these thin films to the film base and overlying layers is poor.
U.S. Pat. No. 4,203,769 describes an antistatic layer incorporating "amorphous" vanadium pentoxide. This vanadium pentoxide antistat is highly entangled, high aspect ratio ribbons 50-100 .ANG. wide, about 10 .ANG. thick, and 0.1-1 .mu.m long. As a result of this ribbon structure surface, resistivities of 10.sup.6-10.sup.11 .OMEGA. per square can be obtained for coatings containing very low volume fractions of vanadium pentoxide. This results in very low optical absorption and scattering losses. Thus the coatings are highly transparent and colorless. However, vanadium pentoxide is soluble at the high pH typical of film developer solutions and must be overcoated with a nonpermeable barrier layer to maintain antistatic performance after film processing.
It can be seen that a variety of methods have been reported in an attempt to obtain non-brittle, adherent, highly transparent, colorless conductive coatings with humidity independent, film process surviving antistatic performance. The aforementioned prior art references relate to some aspects of the present invention, but, they are deficient with regard to simultaneously satisfying all of the above mentioned requirements.
U.S. Pat. No. 5,340,676 describes conductive layers comprising electrically-conductive fine particles, hydrophilic colloid, and water-insoluble polymer particles. Representative polymer particles described include polymers and interpolymers of styrene, styrene derivatives, alkyl acrylates or alkyl methacrylates and their derivatives, olefins, vinylidene chloride, acrylonitrile, acrylamide and methacrylamide derivatives, vinyl esters, vinyl ethers, or condensation polymers such as polyurethanes and polyesters. The use of a mixed binder comprising the polymer particles mentioned above in combination with a hydrophilic colloid such as gelatin provides a conductive coating that requires lower volume % conductive fine particles compared with a layer obtained from a coating composition comprising the conductive fine particles and water soluble hydrophilic colloid alone. Copending commonly-assigned U.S. Provisional Patent Application Serial No. 60/000236, filed Jun. 15, 1995, describes a further improvement to the '676 patent in that the water-insoluble polymer particles are gelatin-grafted polymer particles. The use of gelatin-grafted polymer particles improves the stability of the coating formulation. U.S. Pat. No. 5,466,567 describes conductive layers comprising electrically-conductive fine particles, hydrophilic colloid, and water-insoluble, precrosslinked gelatin particles. Electrically-conductive layers prepared from coating compositions described in the '676 patent, application Ser. No. 60/000236 and the '567 patent are especially useful when the conductive layer is to be overcoated with a layer containing a hydrophilic colloid.
It is toward the objective of providing a new and improved electrically-conductive layer that is capable of utilizing low volume percentages of the electrically-conductive fine particles that the present invention is directed. Use of such low volume percentages provides improved layer transparency since most of the known electrically-conductive fine particles have a high refractive index or are highly colored. In addition, minimizing the amount of electrically-conductive fine particles incorporated into a dried coating, especially for conductive metal oxide particles, can provide improved physical properties (e.g., freedom from brittleness), reduced cost for the coated layer, and reduced finishing tool wear.